Dew Point Calculator

How to Use the Dew Point Calculator

1. 1. Enter the air temperature and choose °C, °F or K. Use the dry-bulb temperature — an ordinary thermometer in the shade.
2. 2. Enter the relative humidity as a percentage between 0 and 100. It must be above zero: completely dry air has no dew point.
3. 3. Read the dew point. The highlighted card is the temperature to which the air must cool, at constant pressure, before dew forms. The frost point appears when it is at or below freezing.
4. 4. Check the working. The steps list the saturation vapor pressure, the vapor pressure and the final inversion, and the table shows how the dew point changes with humidity at your temperature.

What the Dew Point Tells You

Relative humidity is a ratio, so it changes whenever the temperature does — even if no water enters or leaves the air. The dew point does not. It is the temperature at which the air you have is exactly saturated, so it measures how much water vapor is actually there. A cool morning at 90 % humidity and a hot afternoon at 40 % humidity can share the same dew point, and the same amount of moisture in the air.

That makes the dew point a better guide to how the weather feels than humidity. Many National Weather Service offices use a simple rule of thumb: at or below 55 °F (13 °C) the air feels dry, between 55 and 65 °F it turns sticky, and at 65 °F (18 °C) or above it is oppressive. Overnight, once the temperature falls to the dew point, dew, frost or fog forms; forecasters watch the gap between the two, the dew point depression, to judge when.

The Magnus Formula

The calculator first finds the saturation vapor pressure at the air temperature t (in °C) with the improved Magnus form of Alduchov and Eskridge (1996): es = 6.1094 × exp(17.625 t / (243.04 + t)) hPa. The actual vapor pressure is e = RH/100 × es. Solving the same equation for the temperature at which e would saturate the air gives the dew point: Td = 243.04 × ln(e / 6.1094) / (17.625 − ln(e / 6.1094)). These coefficients are fitted for −40 to 60 °C with an error of about 0.4 % in vapor pressure, which moves the dew point by less than 0.1 °C. Other published sets, such as Sonntag's 17.62 and 243.12, agree with these to within about 0.02 °C across ordinary weather.

The frost point is the same calculation with the constants for saturation over ice (6.1121, 22.587 and 273.86). Below freezing, ice holds less vapor than supercooled water at the same temperature, so the frost point is a little higher than the dew point — frost can form while the air is still unsaturated with respect to water. The calculator shows it only when it is at or below 0 °C.

For a quick estimate in your head, Lawrence (2005) showed that for humidity above 50 % the dew point is about Td ≈ t − (100 − RH) / 5 in °C. For 25 °C and 60 % that gives 17 °C, close to the exact 16.7 °C.

Frequently Asked Questions

How do you calculate dew point from temperature and humidity?

Find the saturation vapor pressure at the air temperature, multiply it by the relative humidity to get the actual vapor pressure, then find the temperature at which that vapor pressure would be saturated. The Magnus formula does all three steps in closed form, and the working above shows them with numbers.

What is a comfortable dew point?

Most people find dew points below about 55 °F (13 °C) comfortable, 55–65 °F (13–18 °C) increasingly sticky, and 65 °F (18 °C) or above oppressive. These are rules of thumb, and tolerance varies with acclimatization and activity.

Can the dew point be higher than the temperature?

Not in ordinary air. At 100 % relative humidity the dew point equals the temperature, and below that it is lower. A value above 100 % means supersaturated air, which only lasts briefly, for example inside a cloud or a fog forming.

What is the difference between dew point and frost point?

The dew point is the temperature at which the air becomes saturated with respect to liquid water; the frost point is the temperature at which it becomes saturated with respect to ice. Above freezing only the dew point matters; below 0 °C the frost point is slightly higher, which is why frost can form before the air reaches its dew point.

How accurate is the Magnus formula?

Within its fitted range of −40 to 60 °C the Alduchov and Eskridge coefficients reproduce saturation vapor pressure to about 0.4 %, which moves the dew point by less than 0.1 °C at normal temperatures. Outside that range the calculator warns you that the result is a rougher estimate.

Why does the dew point matter more than humidity?

Relative humidity depends on temperature, so it climbs every night and falls every afternoon without any change in the moisture content. The dew point stays nearly constant through the day unless a new air mass arrives, which makes it the better measure of how muggy the air is and of when fog or dew will form.